<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        ol{
            user-select: none;
            list-style: none;
        }
    </style>
</head>
<body>
    <ol>
        <button>1</button>
        <button>2</button>
        <button>3</button>
        <button>4</button>
        <button>5</button>
    </ol>
    
</body>
<script>
    let btns = document.getElementsByTagName('button');
    for(let i=0;i<btns.length;i++){
        btns[i].onclick = function(){
            for(let j=0; j<btns.length; j++){
                btns[j].style.backgroundColor = '';
            }
            this.style.backgroundColor = 'pink';
        }
    }
</script>
</html>